Method for preparing acetic acid and or methyl acetate in the presence of iridium and platinum

ABSTRACT

The invention relates to a process for the preparation of acetic acid and/or methyl acetate in the liquid phase by the carbonylation of methanol and/or the isomerization of methyl formate in the presence of water, a solvent, a homogeneous catalyst system comprising iridium and a halogen-containing promoter, and carbon monoxide, wherein said catalyst system also comprises platinum.

BACKGROUND OF THE INVENTION

The present invention relates to a process for the preparation of aceticacid and/or methyl acetate in the liquid phase in the presence of ahomogeneous catalyst comprising iridium and platinum.

The process of the invention makes it possible to obtain an increasedproductivity in terms of acetic acid, as well as an improved stabilityof the catalyst system employed.

Various ways of obtaining acetic acid are known and exploited inindustry. These include the methanol carbonylation reaction carried outin the liquid phase, under a pressure of carbon monoxide, which is oneof the reactants, in the presence of a homogeneous catalyst system.Another way of obtaining acetic acid consists in isomerizing methylformate. This reaction, too, is generally carried out in the presence ofa catalyst system in the homogeneous phase. Finally, another processinvolves carrying out the carbonylation of methanol and theisomerization of methyl formate simultaneously.

More precisely, the carbonylation process using rhodium is a knownprocess, exploited in industry, which has formed the subject of numerousarticles and patents, for example American patents U.S. Pat. No.3,769,329 and U.S. Pat. No. 3,813,428.

European patents EP 618 183 and EP 618 184, and European patents EP 785919 and EP 759 022, describe a carbonylation process carried out in thepresence of a catalyst system based on iridium and, if appropriate, alsocontaining rhodium.

A carbonylation process using iridium and ruthenium, which is currentlyexploited in industry, is described in European patent EP 643 034.

More recently, a new preparative process, consisting of a methyl formateisomerization reaction in the presence of iridium, was proposed inFrench patent FR 2 746 794 and international patent application WO97/135829.

In parallel, a process for the preparation of acetic acid and/or methylacetate by carrying out a methyl formate isomerization reaction and amethanol carbonylation reaction simultaneously was proposed in patent FR2 746 795 and international patent application WO 97/35828.

These different processes for acetic acid production are generallycarried out continuously in plants comprising essentially three zones.The first corresponds to the actual reaction zone comprising apressurized reactor in which the carbonylation and/or isomerization arecarried out. The second consists of a zone for separation of the acidformed. This operation is effected by partial vaporization of thereaction mixture in a so-called flash apparatus, in which the pressureis kept below that in the reactor. The vaporized part is then sent to athird zone, where the acetic acid produced is purified. This zonecomprises various distillation columns in which the acetic acid producedis separated from the water, the reactants and the by-products. Thatpart of the mixture which remains in liquid form at the outlet of thevaporization zone, and comprises especially the catalyst, is recycled tothe reactor.

SUMMARY OF THE INVENTION

The aim of the process according to the invention is to improve theprocesses described above by using a homogenous phase catalyst systemcomprising iridium and platinum.

In fact, it has now been found that the addition of platinum to acatalyst system based on iridium or iridium and rhodium makes itpossible, totally unexpectedly, to increase the acid production rate. Inother words, the process according to the invention makes it possible toachieve a reaction rate which is greater than or equal to that obtained,under the same conditions, with a catalyst system involving only iridiumor an iridium/rhodium mixture, while the total number of moles of metalsused in the process of the invention remains unchanged.

Apart from the productivity increase, obtaining comparable rates using asmaller number of moles of catalyst constitutes an additional economicadvantage, namely that of reducing the catalyst costs.

It has also been shown that platinum considerably enhances the stabilityof the iridium, even when the water content is low.

DETAILED DESCRIPTION OF THE INVENTION

The process according to the invention is carried out in the liquidphase. Consequently the catalyst system used is in a form soluble in thereaction medium.

The catalyst system suitable for carrying out the invention is based onat least one iridium compound, by itself or in the presence of rhodium,and at least one halogen-containing promoter and also comprises at leastone platinum derivative.

Thus, according to one of its essential characteristics, the inventionrelates to a process for the preparation of acetic acid and/or methylacetate in the liquid phase by the carbonylation of methanol and/or theisomerization of methyl formate in the presence of water, a solvent, ahomogeneous catalyst system comprising iridium and a halogen-containingpromoter, and carbon monoxide, wherein said catalyst system alsocomprises platinum.

The invention therefore consists in improving the processes for thepreparation of acetic acid by isomerization, by carbonylation or by acombination of isomerization and carbonylation by using a catalystsystem based on soluble iridium and a halogen-containing promoter, andby adding platinum to said catalyst system in a form soluble in themedium.

In a first variant of the process of the invention, methanol iscarbonylated by maintaining a carbon monoxide partial pressure ofbetween 0.1.10⁵ Pa and 200.10⁵ Pa throughout the reaction.

In a second variant of the process, methyl formate is isomerized bymaintaining a carbon monoxide partial pressure of between 0.1.10⁵ Pa and25.10⁵ Pa throughout the reaction. The preferred conditions for carryingout such a process can be obtained directly by those skilled in the artby reference to international patent application WO 97/35829 citedabove, except that platinum is added to the iridium.

In a third variant of the process, a methanol carbonylation reaction anda methyl formate isomerization reaction are carried out simultaneouslyby maintaining a carbon monoxide partial pressure of between 0.1.10⁵ Paand 25.10⁵ Pa throughout the reaction. The preferred conditions forcarrying out such a process can be obtained directly by those skilled inthe art by reference to international patent application WO 97/35828cited above, except that platinum is added to the iridium

In the catalyst system used in the above three variants, the iridium mayadvantageously be replaced with a combination of iridium+rhodium.

In cases where a catalyst system containing rhodium is employed, theatomic ratio of rhodium to iridium may vary within wide limits, i.e.between 0.01 and 99.

In all these variants, the platinum will be introduced into the reactionmedium in a sufficient amount and in appropriate proportions relative tothe iridium. Experiments carried out by the inventors of the presentinvention have in fact shown that the optimum amounts and proportions ofplatinum are intimately associated with the form in which the platinumis introduced into the reaction medium.

In precise terms, it is possible to use any platinum compounds which aresoluble or capable of being solubilized in the reaction medium under theconditions of the invention.

As examples, and without implying a limitation, the following areparticularly suitable for carrying out the invention:

platinum compounds hereafter referred to as “simple compounds”, such asplatinum in the metallic state, its salts and its oxides; and

coordination complexes of this metal.

The compounds in the form of complexes prove to be the preferredcompounds according to the invention.

The salts used are preferably platinum halides. The halogen is moreparticularly selected from chlorine, bromine and iodine, the last ofthese being preferred.

Thus compounds such as PtI₂, PtBr₂, PtCl₂, PtCl₄.xH₂O, H₂PtCl₆.xH₂O,Na₂PtCl₄.xH₂O and Na₂PtCl₆.xH₂O can be used in the process according tothe invention.

Oxides, selected from PtO, PtO₂ and PtO₂.xH₂O, can likewise beappropriately used in the process according to the invention.

As far as the soluble platinum coordination complexes are concerned, themost commonly used compounds are those having ligands selected fromcarbon monoxide and a carbon monoxide/halogen combination, the halogenbeing selected from chlorine, bromine and, more particularly, iodine. Itis not excluded, however, to use soluble platinum complexes whoseligands are selected for example from organophosphorus andorganonitrogen compounds.

The following compounds may be mentioned, without implying a limitation,as coordination complexes which are known to those skilled in the artand are particularly suitable for carrying out the invention: PtI₂(CO)₂,[PtI₂(CO)]₂, [Pt₃(CO)₆]²⁻[Q⁺]₂ and [Pt₆(CO)₁₂]²⁻[Q⁺]₂, in which formulaeQ can be especially hydrogen or a group NR₄ or PR₄, where R is selectedfrom hydrogen and/or a hydrocarbon radical,tetraiododicarbonyldiplatinum, [PtI₂(CO)]₂, being preferred.

In a first variant of the invention, if the platinum is introduced inthe form of a simple compound—platinum in the metallic state, salts oroxides—it will be preferable to maintain a platinum content of at least4 mmol/l of reaction medium and an atomic ratio of iridium to platinumof between 2 and 5.

In a second, particularly preferred variant of the invention, if theplatinum is introduced in the form of a coordination complex of thismetal with ligands selected from carbon monoxide, a carbonmonoxide/halogen combination and organo-nitrogen and organophosphoruscompounds, it will be preferable to maintain a platinum content of atleast 1 mmol/l of reaction medium and an atomic ratio of iridium toplatinum of between 1 and 5.

The atomic ratio of iridium to platinum or, in the case where thecatalyst system also comprises rhodium, the atomic ratio of(iridium+rhodium) to platinum is between 2 and 5 when the platinum isused in the form of simple compounds as defined above, and between 1 and5 when the platinum is introduced in the form of coordination complexes.The platinum content will preferably be at least 4 mmol/l of reactionmedium in the case of simple platinum compounds and at least about 1mmol/l in the case of coordination complexes.

In general, the concentration of iridium or, if appropriate,iridium+rhodium in the reaction medium is between 0.1 and 100 mmol/l,preferably between 1 and 20 mmol/l.

Any of the rhodium- and iridium-based compounds conventionally used incarbonylation and/or isomerization reactions can be employed in theprocess according to the invention.

It is possible to use any iridium compounds which are soluble or capableof being solubilized in the reaction medium under the operatingconditions of the invention. As examples, and without implying alimitation, iridium in the metallic state, salts of this metal oxides orcoordination complexes are particularly suitable for carrying out theinvention.

The iridium salts conventionally used are iridium halides. The halogenis more particularly selected from chlorine, bromine and iodine, thelast of these being preferred. Thus compounds like IrI₃, IrBr₃, IrCl₃,IrI₃.4H₂O, IrI₄ and IrBr₃.4H₂O can be used in the process according tothe invention.

Oxides, selected from IrO₂ and Ir₂O₃.xH₂O, can likewise be appropriatelyused in the process according to the invention.

As far as soluble iridium coordination complexes are concerned, the mostcommonly used compounds are those having ligands selected from carbonmonoxide and a carbon monoxide/halogen combination, the halogen beingselected from chlorine, bromine and, more particularly, iodine. It isnot excluded, however, to use soluble iridium complexes whose ligandsare selected for example from organophosphorus and organonitrogencompounds.

The following compounds may be mentioned, without implying a limitation,as coordination complexes which are known to those skilled in the artand are particularly suitable for carrying out the invention: Ir₄(CO)₁₂,Ir(CO)₂I₂ ⁻Q⁺, Ir(CO)₂Br₂ ⁻Q⁺ and Ir(CO)₂Cl₂ ⁻Q⁺, in which formulae Qcan be especially hydrogen or a group NR₄ or PR₄, where R is selectedfrom hydrogen and/or a hydrocarbon radical.

These catalysts can be obtained by any method known to those skilled inthe art. Reference may thus be made to patents EP 657 386 and EP 737 103for the preparation of iridium-based catalyst solutions suitable forcarrying out the present invention.

As indicated previously, the reaction according to the invention can becarried out with a catalyst system comprising iridium by itself, oriridium and rhodium, in addition to platinum.

In general, the rhodium- and iridium-based compounds used are selectedfrom coordination complexes of these metals which are soluble in themedium under the reaction conditions. More particularly, thecoordination complexes used are those whose ligands are on the one handcarbon monoxide and on the other hand a halogen such as chlorine,bromine or, more particularly, iodine. It is of course possible to usesoluble complexes comprising ligands other than those mentioned, such asorganophosphorus or organonitrogen ligands in particular.Advantageously, however, the present invention does not require the useof rhodium and iridium complexes of this type.

Thus complexes of the type Ir₄(CO)₁₂, Ir(CO)₂I₂ ⁻Q⁺, Ir(CO)₂Br₂ ⁻Q⁺,Rh₄(CO)₁₂, Rh(CO)₂I₂ ⁻Q⁺ or Rh(CO)₂Br₂ ⁻Q⁺, or complexes based on bothmetals, such as Rh₃Ir(CO)₁₂ or Rh₂Ir₂(CO)₁₂, in which formulae Q can beespecially hydrogen or a group NR₄ or PR₄, where R is selected fromhydrogen and/or a hydrocarbon radical, may be mentioned especially asexamples of coordination complexes which are used more particularly inthe present invention.

Compounds selected from the salts of these elements, such as especiallyIrI₃, IrBr₃, IrCl₃, IrI₃.4H₂O, IrBr₃.4H₂O, RhI₃, RhBr₃, RhCl₃, RhI₃.4H₂Oand RhBr₃.4H₂O, or rhodium and iridium in the metallic state, canlikewise be used in the process according to the invention.

It should be noted that the above-mentioned list of rhodium- andiridium-based compounds cannot be considered as exhaustive and thatreference may be made to patents U.S. Pat. No. 3,769,329 and U.S. Pat.No. 3,772,380, whose teaching is included herewith, for additionalexamples of compounds of the two metals mentioned above.

In general, the rhodium-, iridium- and platinum-based compounds used areselected from the coordination complexes of these metals which aresoluble in the medium under the reaction conditions. More particularly,the coordination complexes used are those whose ligands are on the onehand carbon monoxide and on the other hand a halogen such as chlorine,bromine or, more particularly, iodine. It is of course possible to usesoluble complexes comprising ligands other than those mentioned, such asorganophosphorus or organonitrogen ligands in particular. Complexes oftwo or three of the above metals may also be mentioned;[PtRh₅(CO)₁₅]⁻Q⁺, Q being as defined above, may be mentioned withoutimplying a limitation.

One important characteristic of the invention lies in the fact that theplatinum is present in the reaction medium in a sufficient amount and inappropriate proportions relative to the iridium. The platinumconcentration is equal to at least 4 mmol/l of reaction medium and, inaddition, the atomic ratio of iridium or (iridium+rhodium) to platinumis between 2 and 5 in the case where simple platinum compounds are used.If the platinum is introduced in the form of a coordination complex, theplatinum concentration is preferably equal to at least 1 mmol/l ofreaction medium and the atomic ratio of iridium to platinum is between 1and 5. It has in fact been observed, totally unexpectedly, that suchconditions allow a considerable increase in the reaction rate.Remarkably, the stability of the catalyst system is also appreciablyimproved under these conditions.

Apart from the compounds mentioned above, the catalyst system accordingto the invention comprises a halogen-containing promoter. This can takethe form of a halogen by itself or a halogen combined with othermoieties such as hydrogen or the methyl or acetyl radical.

The halogen is generally selected from chlorine, bromine and iodine,iodine being preferred.

Iodine, hydriodic acid, methyl iodide and acetyl iodide may be mentionedas halogen compounds which can also be used as promoters.

Methyl iodide will preferably be used as the halogen-containingpromoter.

In one variant of the invention, the halogen-containing promoter ispartially or totally introduced into the reaction mixture in the form ofa precursor. In such a case, said precursor generally takes the form ofa compound which is capable of releasing the hydrocarbon radical of theabove-mentioned halogen-containing promoter into the reaction mediumunder the action of a halogen or a hydrohalic acid in particular, thelatter compounds being present in the medium or introduced for thispurpose.

Compounds selected from methanol, dimethyl ether, methyl acetate andmethyl formate, used by themselves or in a mixture, may be mentioned asnon-limiting examples of suitable precursors.

The amount of halogen-containing promoter present in the reactionmixture is advantageously less than or equal to 20%, based on the totalweight of said mixture. The content of halogen-containing promoter ispreferably less than or equal to 15%.

It should be noted that if the above-mentioned promoter is partially ortotally introduced in the form of a precursor, the amount of precursoror promoter/precursor mixture is such as to give an amount equivalent tothat mentioned above.

The process of the invention can be carried out by feeding the reactorwith methanol as the only reactant in the case of carbonylation only. Itcan also be carried out by feeding the reactor with methyl formate inthe case of an isomerization process, or with methyl formate andmethanol in the case of a combined process involving simultaneousisomerization and carbonylation.

The reaction according to the invention is also carried out in thepresence of water. The process according to the invention makes itpossible to obtain a good productivity with low water contents, withoutloss of catalyst metal by precipitation.

Thus the process forming the subject of the invention can be carried outover a wide range of water concentrations in the reaction medium, butpreferably with a concentration less than or equal to 14%, based on thetotal weight of said medium. More particularly, the water content of thereaction medium is less than or equal to 10%.

In the first variant of the process, where only the carbonylation ofmethanol is carried out, the water content is preferably between 2% and8% by weight of the reaction medium.

In the second variant, where the isomerization of methyl formate iscarried out, optionally simultaneously with the carbonylation ofmethanol, the water content is less than 5% and preferably less than 2%by weight of the reaction medium.

The process according to the invention can be carried out in thepresence of iodides in a form soluble in the reaction medium. Theiodides can be introduced into the reaction medium as such, but also inthe form of compounds capable of forming soluble iodides.

Iodides are understood as meaning ionic species, i.e. excluding covalentiodides (such as the halogen-containing promoter in particular) andhydriodic acid.

Thus the iodides introduced as such into said mixture are selected frominorganic and organic iodides.

Inorganic iodides which may principally be mentioned are the iodides ofalkaline earth metals or alkali metals, the latter being preferred.Potassium iodide, lithium iodide and sodium iodide may be mentionedamong said alkali metal iodides.

Organic iodides which may be mentioned are organic compounds comprisingat least one organophosphorus group and/or at least one organonitrogengroup and reacting with iodine-based compounds to give ionic speciescontaining this halogen. Examples which may be mentioned are thecompounds of the formula Q⁺.I⁻, in which Q is a group NR₄ or PR₄, whereR is selected from hydrogen and/or a hydrocarbon radical.

Examples which may be mentioned of compounds capable of forming iodidessoluble in the reaction medium are alkali metal or alkaline earth metalcarboxylates and hydroxides, such as lithium acetate, potassiumhydroxide and sodium hydroxide in particular.

It should moreover be noted that the iodides may originate in ways otherthan those indicated above.

These compounds can thus originate from impurities, such as alkalimetals or alkaline earth metals, present in the starting materialsemployed to prepare the catalyst solution.

Likewise the iodides can originate from the corrosion metals appearingduring the reaction. It is preferable to keep the concentrationthreshold of these metals relatively low, of the order of a few hundredparts per million, because they have the effect especially of favoringthe water-gas shift reaction and contribute to increasing the atomicratio of iodides to iridium.

It is possible to introduce a particular amount of iodides into thereaction medium according to the amount of iridium present in themedium. Thus said amount of iodides introduced is such that the atomicratio of iodides introduced to iridium (expressed in mol/mol) is below10 and is kept within this range throughout the reaction.

In one preferred embodiment of the invention, the atomic ratio ofiodides to iridium is kept below 3. More particularly, this ratio isbelow 1.5.

It has been found that the addition of such amounts of iodides makes itpossible to improve the stability of the catalyst and keep theproductivity of the process high.

Consequently the present invention is more particularly intended to becarried out continuously and the stable operating conditions of theprocess correspond to the composition and proportions indicated.

More particularly, as far as the soluble iodides are concerned, theatomic ratio of soluble iodides to iridium can be maintained by treatinga mixture comprising at least the iridium compound with an ion exchangeresin and then adding iodides in soluble form in an amount such thatsaid ratio is below 10.

In addition to the compounds and reactants mentioned above, the reactionaccording to the invention is advantageously carried out in the presenceof esters.

The ester used is preferably methyl acetate and/or methyl formate, whichare used as such or in a masked form.

In one embodiment of the invention, the ester content is advantageouslyless than or equal to 40% by weight, based on the weight of the reactionmixture. More particularly, this content remains less than or equal to30%.

Finally, the reaction is carried out in the presence of a solvent. Thesolvent used in the process according to the invention is advantageouslyacetic acid or formic acid. It is of course possible to use othersolvents such as, in particular, compounds which are inert towards thereaction mixture and have a boiling point above that of the acid formed.

The reaction is generally carried out at a temperature between 150 and250° C. More particularly, the reaction temperature is between 175 and210° C. and preferably between 175 and 200° C.

The total pressure under which the reaction is carried out is generallyabove atmospheric pressure. More particularly, it is advantageouslybelow 200.10⁵ Pa and preferably below or equal to 50.10⁵ Pa. In the caseof a process according to the invention which involves the isomerizationof methyl formate, optionally simultaneously with the carbonylation ofmethanol, the CO partial pressure will advantageously be between 0.1.10⁵Pa and 25.10⁵ Pa, as indicated previously. The pressures are expressedin absolute pascals and are measured hot, i.e. under the temperatureconditions of the reaction.

The process according to the invention is preferably carried out in thepresence of a corrosion metal content of less than a few hundred ppm,preferably of less than 200 ppm. The corrosion metals are especiallyiron, nickel, chromium, molybdenum and zirconium. The corrosion metalcontent of the reaction mixture is maintained by the methods known tothose skilled in the art, for example selective precipitation,liquid—liquid extraction or passage over ion exchange resins.

In general, the process of the invention is advantageously carried outcontinuously.

The reaction is carried out in equipment which is resistant to thecorrosion created by the medium. Thus zirconium or alloys of theHastelloy® C or B type are particularly suitable for the operatingconditions of the reaction.

When the reaction is started, the various components are introduced intoan appropriate reactor, which is fitted with stirring means so as toensure a good homogeneity of the reaction mixture. It should be notedthat although the reactor preferably comprises mechanical means forstirring the reaction mixture, it is not excluded to dispense with suchmeans, it being possible for the mixture to be homogenized by the carbonmonoxide introduced into the reactor.

It should be noted that the reaction could conveniently be carried outin a reactor of the piston type.

It is of course possible to envisage combining several reactors of thestirred and piston type.

Carbon monoxide can be introduced directly into the reactor where thereaction according to the invention takes place, but it can also beintroduced into the recycling zone, which will be described below.

The reaction mixture leaving the reactor is treated in an appropriatemanner for separating the products from the reaction mixture comprisingespecially the catalyst.

For this purpose, and in the case where the reaction is carried outcontinuously, it is possible for example to employ a conventionaltechnique which consists in expanding the mixture so as to cause apartial vaporization thereof This operation can be effected using avalve for expanding the mixture, the latter being introduced into aseparator (called a flash separator). The operation can take place withor, preferably, without the provision of heat, i.e. under adiabaticconditions.

The non-vaporized part, comprising especially the catalyst which hasremained in solution, is advantageously recycled to the reactor,conventionally by means of a pump.

The vaporized part, which comprises the acetic acid and/or the methylacetate produced, is then sent to a purification zone, whichconventionally comprises various distillation columns.

Finally, the process according to the invention can be carried out withthe insertion of an additional reactor between the main reactor and thepartial vaporization zone, more particularly upstream of the reactionmixture expansion valve; in said additional reactor, the carbon monoxidepresent in the dissolved and/or entrained state will be wholly orpartially consumed.

The preferred conditions for carrying out such a process can be obtaineddirectly by those skilled in the art by reference to patent FR 2 750984, except that, according to the invention, platinum is added to theiridium.

EXAMPLES I—Examples of the Carbonylation of Mixtures of MethylAcetate+Methanol in the Presence of Platinum(II) Iodide

Comparative Examples A, B, C, D and E and Examples 1 and 2 according tothe invention

A series of experiments were carried out which were identical to oneanother in every respect except for the nature and composition of thecatalyst system. The operating conditions of these experiments aredescribed in detail in Comparative Experiment A below and Table no. 1.

The results obtained in the various experiments are collated in Tableno. 2 below, which shows:

-   -   in the column headed V_(carb), the carbonylation rates        calculated on the basis of the CO consumption measured in the        reactor after a reaction time of 10 minutes (RT=10),        corresponding to the amount of acetic acid formed by        carbonylation during this period; V_(carb) is expressed in        mol/l.h;    -   in the column headed TOF (=Turnover Frequency), the ratio of the        rate to the total metal concentration; TOF is expressed in h⁻¹.        Comparative Experiment A: Carbonylation Reaction in the Presence        of Iridium by Itself

First of all, the catalyst solution is prepared as follows:

The following are introduced into a 100 ml HASTELLOY® B2 autoclave:

-   -   0.454 g of iridium iodide,    -   10 g of acetic acid,    -   1 g of water.

The autoclave is then placed under a carbon monoxide absolute pressureof 6 bar at room temperature.

The temperature is raised to 190° C.

The preparation of the catalyst solution takes 25 minutes.

The carbonylation reaction is carried out as follows:

Acetic acid, methyl iodide, water, methanol and methyl acetate areinjected into the autoclave under CO pressure.

The initial composition of the reaction mixture is as follows (byweight):

-   water: 6.4%-   methyl acetate: 30%-   methyl iodide: 10%-   methanol: 5.7%-   iridium: 1943 ppm-   acetic acid: qsp 100%

The total absolute pressure is kept constant at 30 bar by injectingcarbon monoxide.

The temperature is maintained at 190° C.±0.5° C.

After the reaction, the reaction liquid weighs 72 g.

The carbonylation rate (V_(carb)) is 11 mol/l/h.

TOF is 1030 h⁻¹.

After the reaction, a light metal deposit is observed in the autoclave.

Comparative Experiments B, C, D and E

Experiments no. 1 and 2 According to the Invention

These experiments are carried out under the same operating conditionsand with the same initial composition of the reaction mixture except forthe catalysts, the details of which are given in Table no. 1, thecatalysts being introduced in the form of iridium iodide and/or rhodiumiodide and/or platinum(II) iodide.

TABLE NO. 1 CATALYSTS Observations at RT = 10 minutes Iridium RhodiumPlatinum Total catalyst Platinum Weight of Volume of iodide iodideiodide content concentration reaction liquid reaction liquid EXPERIMENT(g) (g) (g) mg/kg (ppm) (mmol/l) (g) (ml) A (Comparative) 0.454 0 0 19430.0 72 68 B (Comparative) 0.782 0 0 3243 0.0 74 70 C (Comparative) 0.2560.473 0 2392 0.0 75 70 D (Comparative) 0 0 0.48 3114 16.2 67 66 E(Comparative) 0 1.2 0 3361 0.0 76 71 1 0.547 0 0.144 3163 4.7 73 68 20.454 0 0.096 2487 3.1 73 69 Initial composition: water = 6.4%, AcOMe =30%, MeI = 10%, MeOH = 5.7%, catalysts according to Table, AcOH = qsp100%, T = 190° C., P total = 30 bar

TABLE 2 Metal deposit in V (carb) at TOF at autoclave Rh/lr Ir/Pt RT =10 minutes RT = 10 minutes EXPERIMENT at RT = 10 minutes (mol/mol)(mol/mol) (mol/h.l) (h⁻¹) A (Comparative) Light deposit 0/100% 100/0% 111030 B (Comparative) Deposit 0/100% 100/0% 18 1010 C (Comparative)Deposit 70/30% 100/0% 17 860 D (Comparative) Heavy deposit  0/0% 0/100%0 0 E (Comparative) Heavy deposit 100/0%  0/0% 16.5 470 1 No deposit0/100% 73/27% 21 1190 2 Traces of deposit 0/100% 75/25% 10 730

As is clearly apparent from Table 2, all the experiments described abovedemonstrate the following points:

Platinum by itself, in the form of platinum(II) iodide, PtI₂, has nocatalytic action in the carbonylation of methanol (Experiment D).

When used in a sufficient amount and in appropriate proportions relativeto the iridium (Experiment 1), platinum increases the catalytic activityof the iridium in the carbonylation of methanol, compared with rhodiumby itself (Experiment E), iridium by itself (Experiments A and B) andiridium+rhodium (Experiment C).

Experiment 2, which was not carried out under the optimum conditions ofthe invention (Pt=3,1 mmol/l), nevertheless shows an improvement in thestability of the catalyst.

The remarkable stability of the catalysts in Experiment no. 1 accordingto the invention, since no metal deposit of iridium and/or platinum isobserved.

Table no. 2 clearly demonstrates the improvement in the carbonylationrate for a sufficient platinum content in the catalyst system and anappropriate atomic ratio of iridium to platinum.

II—Examples of the Carbonylation of Methyl Acetate in the Presence ofTetraiododicarbonyldiplatinum, [PtI₂(CO)]₂

Experiments 3, 4, 5 and 6 According to the Invention ComparativeExperiments F and G

A series of experiments were carried out which were identical to oneanother in every respect except for the nature and composition of thecatalyst system. The operating conditions of these experiments aredescribed in detail in Comparative Experiment G below and Table no. 3.

The results obtained in the various experiments are collated in Tableno. 4, which shows:

-   -   in the column headed V_(carb), the carbonylation rates        calculated on the basis of the CO consumption measured in the        reactor for given concentrations of methyl acetate—AcOMe—of 20%        and 15% by weight in the reaction mixture, corresponding to the        amount of acetic acid formed by carbonylation; V_(carb) is        expressed in mol/l.h;    -   in the column headed TOF (=Turnover Frequency), the ratio of the        rate to the total metal concentration; TOF is expressed in h⁻¹.        Comparative Experiment G: Carbonylation Reaction in the Presence        of Iridium by Itself

First of all, the catalyst solution is prepared as follows:

The following are introduced into a 100 ml HASTELLOY® B2 autoclave:

-   -   0.4596 g of iridium iodide,    -   10 g of acetic acid,    -   1 g of water.

The autoclave is then placed under a carbon monoxide absolute pressureof 6 bar at room temperature.

The temperature is raised to 190° C.

The preparation of the catalyst solution takes 25 minutes.

The carbonylation reaction is carried out as follows:

Acetic acid, methyl iodide, water and methyl acetate are injected intothe autoclave under CO pressure.

The initial composition of the reaction mixture is as follows (byweight):

water: 6.4%  methyl acetate: 30% methyl iodide: 10% methanol:  0%iridium: 2587 ppm acetic acid: qsp 100%

The total absolute pressure is kept constant at 30 bar by injectingcarbon monoxide.

The temperature is maintained at 190° C.±0.5° C.

After the reaction, the reaction liquid weighs 52.3 g.

The carbonylation rate (V_(carb)) is 16 mol/l/h and 14 mol/l/h,respectively, for 20% and 15% by weight of AcOMe.

The corresponding values of TOF are 1110 and 970 h⁻¹.

After the reaction, a metal deposit is observed in the autoclave.

Comparative Experiment F

Experiments no. 3, 4, 5 and 6 According to the Invention

These experiments are carried out under the same operating conditionsand with the same initial composition of the reaction mixture except forthe catalysts, the details of which are given in Table no. 3, thecatalysts being introduced in the form of iridium iodide andtetraiododicarbonyldiplatinum, [PtI₂(CO)]₂.

TABLE NO. 3 CATALYSTS Observations at RT = 10 minutes Iridium RhodiumTotal catalyst Weight of Volume of iodide iodide [PtI₂(CO)]₂ content Ptconcentration reaction reaction EXPERIMENT (g) (g) (g) (ppm) (mmol/l)liquid (g) liquid (ml) F (Comparative) 0 0 0.1662 1399 7.6 48.6 46 G(Comparative) 0.4596 0 0 2587 0.0 52.3 49 3 0.3899 0 0.2491 4143 10.952.3 48 4 0.4600 0 0.1662 3989 7.3 51.0 48 5 0.5001 0 0.0844 3429 3.653.0 49 6 0.2784 0 0.4134 4894 18.1 51.3 48 Initial composition: water =6.4%, AcOMe = 30%, MeI = 10%, MeOH = 0%, catalysts according to Table,AcOH = qsp 100%, T = 190° C., P total = 30 bar

TABLE NO. 4 Metal deposit in V_((carb)) at 20% TOF at 20% V_((carb)) at15% TOF at 15% autoclave at Rh/Ir Ir/Pt [AcOMe] [AcOMe] [AcOMe] [AcOMe]EXPERIMENT RT = 10 minutes (mol/mol) (mol/mol) (mol/h.l) (hour⁻¹)(mol/h.l) (hour⁻¹) F (Comparative) Heavy deposit  0/0% 0/100% 2.5 (at RT= 10) 330 (at RT = 10) G (Comparative) Deposit 0/100% 100/0% 16 1110 14 970 3 No deposit 0/100% 53/47% 33 1410 24 1030 4 No deposit 0/100%67/33% 35 1600   29.5 1340 5 No deposit 0/100% 81/19% 31 1610 24 1250 6No deposit 0/100% 33/67% 24  890 20  740

All the experiments described above demonstrate the following points:

Platinum by itself, employed in the form oftetraiododicarbonyldiplatinum, [(PtI₂(CO)]₂, has a valuable catalyticaction in terms of TOF (Experiment F=330 h⁻¹) compared with platinum byitself in non-carbonylated form (Experiment D=0 h⁻¹).

When used in association with iridium (Experiments no. 3, 4 and 5according to the invention) in a sufficient amount and in appropriateproportions relative to the iridium, platinum in the form oftetraiododicarbonyldiplatinum, [PtI₂(CO)]₂, increases the activity ofthe iridium in the carbonylation of methyl acetate, compared withiridium by itself (Comparative Experiment G).

Experiment 6, which was not carried out under the optimum conditions ofthe invention (Ir/Pt=0.5), nevertheless shows an improvement in thestability of the catalyst.

The remarkable stability of the catalysts in Experiments no. 3, 4 and 5according to the invention, since no metal deposit of platinum and/oriridium is observed.

Table no. 4 clearly demonstrates the improvement in the carbonylationrate for a sufficient platinum content in the catalyst system and anappropriate atomic ratio of iridium to platinum.

1. A process for the preparation of acetic acid, methyl acetate or bothacetic acid and methyl acetate in a liquid phase reaction mediumcomprising isomerization of methyl formate and optionally carbonylationof methanol, in the presence of water, a solvent, a homogeneous catalystsystem comprising iridium and a halogen-containing promoter, and carbonmonoxide, wherein said catalyst system also comprises platinum.
 2. Theprocess as claimed in claim 1, wherein said process is a process ofisomerization of methyl formate wherein a carbon monoxide partialpressure of between 0.1.10⁵ Pa and 25.10⁵ Pa is maintained throughoutthe reaction.
 3. The process as claimed in claim 1 wherein said reactioncomprises a simultaneous methanol carbonylation reaction and a methylformate isomerization reaction, wherein said reaction is carried outunder a carbon monoxide partial pressure of between 0.1.10⁵ Pa and25.10⁵ Pa throughout the reaction.
 4. The process as claimed in claim 1,wherein the platinum is introduced into said catalyst system in the formof metallic platinum, a platinum salt or an oxide.
 5. The process asclaimed in claim 1, wherein the platinum is introduced into the catalystsystem in the form of a coordination complex.
 6. The process as claimedin claim 5, wherein the coordination complex is a coordination complexof platinum with at least one ligand selected from the group consistingof carbon monoxide, a carbon monoxide/halogen combination,organonitrogen compounds and organophosphorus compounds.
 7. The processas claimed in claim 5, wherein said complex is [PtI₂(CO)]₂.
 8. Theprocess as claimed in claim 4, wherein the platinum concentration of atleast 4 mmol/l of reaction medium and an atomic ratio of iridium toplatinum of between 2 and 5 are maintained.
 9. The process as claimed inclaim 5, wherein a platinum content of at least 1 mmol/l of reactionmedium and an atomic ratio of iridium to platinum of between 1 and 5 aremaintained.
 10. The process as claimed in claim 1, wherein said catalystsystem also contains rhodium.
 11. The process as claimed in claim 10,wherein rhodium and iridium are maintained in an atomic ratio of rhodiumto iridium of between 0.01 and
 99. 12. The process as claimed in claim10, wherein a concentration of iridium and rhodium in the reactionmedium of between 0.1 and 100 mmol/l is maintained.
 13. The process asclaimed in claim 10, wherein the platinum is introduced into thecatalyst system in the form of metallic platinum, a platinum salt or aplatinum oxide, and a platinum content of at least 4 mmol/l of reactionmedium and an atomic ratio of (iridium+rhodium) to platinum of between 2and 5 are maintained.
 14. The process as claimed in claim 10, whereinthe platinum is introduced in the form of a coordination complex, and aplatinum content of at least 1 mmol/l of reaction medium and an atomicratio of (iridium+rhodium) to platinum of between 1 and 5 aremaintained.
 15. The process as claimed in claim 1, wherein aconcentration of iridium in the reaction medium of between 0.1 and 100mmol/l is maintained.
 16. The process as claimed in claim 15, wherein aconcentration of iridium in the reaction medium of between 1 and 20mmol/l, is maintained.
 17. The process as claimed in claim 1 which iscarried out in the presence of a water content less than or equal to 14%by weight of the reaction medium.
 18. The process as claimed in claim17, which is carried out in the presence of a water content less than orequal to 10% by weight of the reaction medium.
 19. The process asclaimed in claim 18, in which the reaction medium contains water in anamount of less than 5% by weight.
 20. The process as claimed in claim19, wherein the reaction medium contains water in an amount of less than2% by weight.
 21. The process as claimed in claim 1, wherein saidhalogen-containing promoter comprises an elemental halogen or a halogenin a compound with hydrogen or a methyl or acetyl radical.
 22. Theprocess as claimed in claim 21, wherein said halogen-containing promoteris methyl iodide.
 23. The process as claimed in claim 1, which iscarried out in the presence of a halogen-containing promoter in anamount of less than or equal to 20% by weight of the reaction medium.24. The process as claimed in claim 23, which is carried out in thepresence of a halogen-containing promoter in an amount of less than orequal to 15% by weight of the reaction medium.
 25. The process asclaimed in claim 1, which is carried out in the presence of an ester inan amount of less than 40% by weight of the reaction medium.
 26. Theprocess as claimed in claim 25, which is carried out in the presence ofan ester in an amount of less than 30% by weight of the reaction medium.27. The process as claimed in claim 1, wherein iodides are introducedinto the reaction medium in an amount sufficient to maintain an atomicratio of soluble iodides introduced into the reaction medium to iridiumof less than
 10. 28. The process as claimed in claim 1, which is carriedout continuously.
 29. A process for the preparation of acetic acid,methyl acetate or both acetic acid and methyl acetate by a reaction ofcarbonylation of methanol in a liquid phase reaction medium in thepresence of water, a solvent, a homogeneous catalyst system comprisingiridium and a halogen-containing promoter, and carbon monoxide, whereinsaid catalyst system also comprises platinum.
 30. The process as claimedin claim 29, wherein a carbon monoxide partial pressure of between0.1.10⁵ Pa and 200.10⁵ Pa is maintained throughout the reaction.
 31. Theprocess as claimed in claim 29, wherein the platinum is introduced intosaid catalyst system in the form of metallic platinum, a platinum saltor an oxide.
 32. The process as claimed in claim 29, wherein theplatinum is introduced into the catalyst system in the form of acoordination complex.
 33. The process as claimed in claim 32, whereinthe coordination complex is a coordination complex of platinum with atleast one ligand selected from the group consisting of carbon monoxide,a carbon monoxide/halogen combination, organonitrogen compounds andorganophosphorus compounds.
 34. The process as claimed in claim 32,wherein said complex is [PtI₂(CO)]₂.
 35. The process as claimed in claim31, wherein the platinum concentration of at least 4 mmol/l of reactionmedium and an atomic ratio of iridium to platinum of between 2 and 5 aremaintained.
 36. The process as claimed in claim 32, wherein a platinumcontent of at least 1 mmol/l of reaction medium and an atomic ratio ofiridium to platinum of between 1 and 5 are maintained.
 37. The processas claimed in claim 29, wherein said catalyst system also containsrhodium.
 38. The process as claimed in claim 37, wherein rhodium andiridium are maintained in an atomic ratio of rhodium to iridium ofbetween 0.01 and
 99. 39. The process as claimed in claim 37, wherein aconcentration of iridium and rhodium in the reaction medium of between0.1 and 100 mmol/l is maintained.
 40. The process as claimed in claim37, wherein the platinum is introduced into the catalyst system in theform of metallic platinum, a platinum salt or a platinum oxide, and aplatinum content of at least 4 mmol/l of reaction medium and an atomicratio of (iridium+rhodium) to platinum of between 2 and 5 aremaintained.
 41. The process as claimed in claim 37, wherein the platinumis introduced in the form of a coordination complex, and a platinumcontent of at least 1 mmol/l of reaction medium and an atomic ratio of(iridium+rhodium) to platinum of between 1 and 5 are maintained.
 42. Theprocess as claimed in claim 29, wherein a concentration of iridium inthe reaction medium of between 0.1 and 100 mmol/l is maintained.
 43. Theprocess as claimed in claim 42, wherein a concentration of iridium inthe reaction medium of between 1 and 20 mmol/l, is maintained.
 44. Theprocess as claimed in claim 29, which is carried out in the presence ofa water content less than or equal to 14% by weight of the reactionmedium.
 45. The process as claimed in claim 44, which is carried out inthe presence of a water content less than or equal to 10% by weight ofthe reaction medium.
 46. The process as claimed in claim 44, wherein thereaction medium contains water in an amount of between 2 and 8% byweight.
 47. The process as claimed in claim 29, wherein saidhalogen-containing promoter comprises an elemental halogen or a halogenin a compound with hydrogen or a methyl or acetyl radical.
 48. Theprocess as claimed in claim 47, wherein said halogen-containing promoteris methyl iodide.
 49. The process as claimed in claim 29, which iscarried out in the presence of a halogen-containing promoter in anamount of less than or equal to 20% by weight of the reaction medium.50. The process as claimed in claim 49, which is carried out in thepresence of a halogen-containing promoter in an amount of less than orequal to 15% by weight of the reaction medium.
 51. The process asclaimed in claim 29, which is carried out in the presence of an ester inan amount of less than 40% by weight of the reaction medium.
 52. Theprocess as claimed in claim 51, which is carried out in the presence ofan ester in an amount of less than 30% by weight of the reaction medium.53. The process as claimed in claim 29, wherein iodides are introducedinto the reaction medium in an amount sufficient to maintain an atomicratio of soluble iodides introduced into the reaction medium to iridiumof less than
 10. 54. The process as claimed in claim 29, which iscarried out continuously.